Low profile wideband antennas and arrays are essential for high data rate communications and software defined radios (SDRs). An ultra-wideband (UWB) array replaces several narrowband systems to reduce power, cost, and space requirements. They also enable increased spectral efficiency, Multiple-Input-Multiple-Output (MIMO), and simultaneous transmit and receive (STAR) capabilities. In addition to being wideband and low profile, such arrays must operate across a wide scanning range for comprehensive spatial coverage.
UWB phased array design is often hindered by the size-bandwidth-scanning triad. For example, Vivaldi or tapered slot arrays are known for their large operational bandwidth, viz. 10:1 impedance bandwidth, but are multiple wavelengths tall. Among low profile UWB arrays, the Tightly Coupled Dipole Antenna (TCDA) array has demonstrated large impedance bandwidths and scanning performance in a low profile of (λHigh/2). These UWB arrays are extensions of the Current Sheet Array (CSA) concept. The first CSAs achieved 4:1 bandwidth by introducing capacitive coupling between antenna elements to counter the effect of ground plane inductance. Additional bandwidth was later achieved by introducing integrated wideband printed balun feeds to be optimized along with the dipole elements. Such TCDA with integrated feeds have been demonstrated to extend bandwidths, reduce size by more than half, and cut weight by a factor of 5, all with an order of magnitude cost reduction. Further optimizations of the TCDA were addressed to increase impedance bandwidths up to 20:1 via substrate loading, scan down to 75° through Frequency Selective Surface (FSS) superstrates, and operate at millimeter-wave frequencies. As a result, TCDAs were designed from 300 MHz up to 90 GHz with VSWR <3.
All the above-mentioned designs employ wideband single-ended (unbalanced) feeds, but these feeds are not suited for the direct chip integration required for 5G applications. The latter is important as future integrated transceivers are likely to be differential to accompany the balanced transmission lines on the RF side of the chips. The major challenge in the design of a full differential radio is the reduction of the common mode currents that can exist at the aperture and in between the ports that feed the aperture. These common mode currents can greatly reduce the impedance bandwidth. Indeed, differential feeds have been proposed in the past, but they are narrowband with limited scanning capability. Therefore, most past arrays have employed only single-ended feeds to achieve wideband scanning. However, these single-ended feeds suffer from distortions introduced by noise from common-mode, power supplies, or general electromagnetic interference (EMI), drastically affecting antenna performance.
Differential Radio Frequency (RF) front-ends provide greater immunity to ground noise and distortion by suppressing external interference. Recent advancements in differential RF front-ends offer high dynamic range, high linearity, and low noise in the transceiver chain. However, a major bottleneck with differential systems is the presence of a common mode when operating across large bandwidths, particularly for wideband phased arrays.